Photoconductive layer construction



United States Patent 3,481,271 PHOTOCONDUCTIVE LAYER CONSTRUCTION Ken-Ichi Shimazu, Yonkers, N.Y., assignor to Polychrome Corporation, Yonkers, N.Y. No Drawing. Filed Mar. 17, 1967, Ser. No. 623,814 Int. Cl. B41n 1/04; G03g 5/04 U.S. Cl. 101-453 18 Claims ABSTRACT OF THE DISCLOSURE There is disclosed in this application an electroconductive, zinc oxide coated :base of flexible material suitable for electrostatic sensitization and subsequent electrophotographic exposure forming a charged latent image that can be developed with an oleophilic resinous material and whose non-image areas can be converted to hydrophilic zinc oxide reaction products to form a planographic printing plate, the zinc oxide being in a dual coating in the second, or uppermost, layer of which the concentration of dispersed zinc oxide in an insulating binder medium is greater than its concentration in the first, or lower, coating layer on the base.

This invention relates to an article capable of being transformed into a planographic printing plate, and more particularly to a treated photoconductive master sheet that can be converted electrophotographically into a planographic printing plate.

Technical advances in the progressing art of electrophotography have included the making of printing plates for use in planographic printing processes, such as lithography, directly from original documents such as books, correspondence and drawings. In one process a planographic plate is indirectly prepared on a paper base by deposition and fusing on the base of a powder image from a photoconductive plate or drum which, after :being exposed to the image to be reproduced, bears a charged latent image that has been dusted with a charged powder. The fused powder is oleophilic, and the plate is made ready for printing by a final treatment of its surface with a wetting solution to prepare the non-image, non-printing areas thereon. In a more direct process a photoconductively prepared sheet, for example, one having a paper base usually treated with an organic or inorganic material to increase its electrical conductivity and coated with a photoconductive substance such as a dyed zinc oxide powder, is exposed electrophotographically to the image to be reproduced, for example, -by sensitizing the sheet with a corona discharge and then exposing the sensitized sheet to light through an image transparency or the like. The latent image thus imparted to the master is converted, or developed, to an oleophilic visible image by either of two common developing techniques, one of which utilizes a dry medium and the other a wet medium. Final preparation includes desensitizing and conversion of the nonimage oleophilic photoconductive areas to a hydrophilic form. A number of patents provide technical background in the art of making planographic printing plates electrophotographically, including U.S. Patents No. 2,952,536, No. 2,957,765, No. 3,001,872, No. 3,106,158, No. 3,128,204 and No. 3,220,830.

Factors affecting the quality of planographic printing plates produced by those methods, and printed materials reproduced therewith, include the density of the image, which in general is preferably high, clarity of the background and its freedom from tinting or darkening in the non-image areas, ease of development and conversion, and press life, i.e., durability on a press during actual printing.

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It is an object of this invention to provide improved planographic printing plates prepared by electrophotographic means. It is also an object of this invention to provide an improved photoconductively sensitive master suitable for being made into a planographic printing plate by exposure with electrophotographic means. Another object of this invention is to provide an improved electrophotographically sensitive planographic master adapted particularly for use in conjunction with machines embodying direct electrophotographic processes. Still another object of this invention is to provide an electrophotographically prepared planographic printing plate that is easily developed and has improved image density, background clarity and durability on the press. These and other objects of this invention will be in part discussed and in part apparent in the more detailed disclosure of the invention hereinbelow.

In a broad aspect the masters of this invention comprise a base sheet of an electroconductive flexible material having thereon a first photoconductive coating of powdered zinc oxide dispersed in a resinous insulating binder medium and, on top of that coating, a second photoconductive coating of powdered zinc oxide dispersed in a resinous insulating binder medium, the concentration of the zinc oxide dispersed in the second coating being substantially greater than the corresponding concentration in the first coating. A latent image is created on the zinc oxide coating by means of an electrophotographic device. In such a device the surface of the zinc oxide coating is electrostatically sensitized with an electrical charge, for example, a negative charge by a corona discharge, and then the coated sensitized sheet is exposed to light which discharges the charge on the zinc oxide coating in the areas exposed to the light that correspond to non-image areas, and leaves charged areas that correspond to the desired image. That latent image is developed by application of an oleophilic resinous material, which will become the ink-receptive image-printing area during printing with the plate, either in the form of a dry charged powder or from a dispersion in a liquid medium. For convenience so the image will be visible it is desirable to color the resinous material by means of a coloring agent such as a pigment or a dye.

Developing compositions, often called toners, are available commercially, for example, a dry toner, Cat. No. 32-430 from Chas. Bruning Company, and a liquid developer, Microstatic from SCM Corporation. Others are described in U.S. Patents No. 2,638,416, No. 2,689,- 670, No. 2,907,674, No. 3,058,914, No. 3,128,204 and No. 3,220,830. Thereafter the surface of the oleophilic zinc oxide coating in the non-image areas is converted by chemical reaction to a hydrophilic reaction product, and the planographic printing master is ready for use. Chemicals and compositions containing them, for elfecting conversion of the surface of the zinc oxide coating are described, for example, in U.S. Patents No. 2,952,536, No. 2,957,765, No. 3,001,872 and No. 3,106,158, and include particularly phosphates, ferrocyanides, ferricyanides, cobalticyanides, vanadates, molybdates, tungstates, tartarates, oxalates, tannates and citrates.

Suitable base materials are sheets or films having sufficient flexibility to be processed by machines employing the electrophotographic method and to be used on a planographic press. They should be electroconductive to at least a small extent, and should have a reasonable degree of dimensional stability for the intended use. Such base materials include metals such as steel, copper and aluminum, fibrous substances such as paper, woven textiles, knitted textiles and felts, and resins such as polyesters, polycarbonates, polystyrenes, cellulose acetates, polyvinyl acetates, polyvinyl chlorides, polyethylenes, polyethylene terephthalates, polyamides and the like. Ma-

terials that are not significantly electro-conductive can be treated to increase that property, for example, by impregnation or coating with organic or inorganic substances of relatively low resistivity. Such substances include, for instance, magnesium bromide and magnesium chloride, aplied alone from aqueous solution or together with a binder such as starch, and organic hydrophilic colloids such as polyvinyl alcohol, polyvinyl acetate and carboxymethyl cellulose. It is possible to treat the base sheet to make it electro-conductive after coating it with the two zinc oxide dispersions. Dimensional stability of the base materials also can be improved by coating or impregnating with suitable materials, in certain instances by some of the substances previously mentioned as well as by such things as thermosetting and epoxy resins and the like. A number of commercially available conductivelytreated papers are suitable as base sheet materials. Typical of such papers are those sold by Weyerhaeuser Paper Company and by International Paper Company. Paper bases are presently preferred because they appear to be economically most attractive,

A sheet of the selected base material is coated with two dispersions of powdered zinc Oxide in resinous insulating binder media. By insulating is meant that the binder is substantially non-electroconductive, which helps retain the charge on the surface of the zinc oxide coating. Suitable binders include film-forming natural and synthetic resins or resiaoids that are soluble in water or in organic solvents, or that can be used in the form of aqueous dispersion or emulsion, for example, as disclosed in US. Patent No. 3,128,204. A typical aqueous dispersion binder is Hycar No. l800x72 acrylic copolymer made by B. F. Goodrich Company. Binders in organic solvents include Pliolite S and S-7, styrene-butadiene copolymers sold by Goodyear Tire and Rubber Company, Epotuf 38-401, epoxy esters sold by Reichhold Chemical Corporation, Cellokyd 6016x, styrenated alkyd resin sold by Cellomer Corporation, and Arotap 2200-X50, alkyd resin sold by ADM Chemicals Company.

factory results, which are usually attainable with coatings of combined thickness in the range of about 5 to microns. The preferred range of thickness is about 8 to 12 microns. Although the thickness of the two coatings is desirably about the same, they can vary within ranges such that the first coating is up to about 80% of the total thickness and the second coating is up to about 90% of the total thickness.

The concentration of zinc oxide dispersed in the binder of the coatings, in terms of ratios of parts by weight, varies considerably. In the first coating it presently appears that zinc oxide concentrations in proportions of about 1:10 to about 8:1 based on the binder can be used. Preferably the concentration of zinc oxide in the first coating should be about 1:1 to about 5:1, and more advantageously about 2:1 to 4:1. In the second coating the suitable zinc oxide concentration proportion ranges from about 2:1 to about 32:1 based on the amount of binder. The preferred concentration is between about 4:1 and about 12:1, and best results as well as economy appear to be attainable at proportions of about 6:1 to 10:1. The concentration of the second coating should be at least about half again that of the first coating, preferably about at least twice as great, and advantageously in the range of about 2 to about 4 times as great.

Optionally, a sensitizer can be included in the zinc oxide-resinous binder coating to improve the photoelectric sensitivity of the coating to electrophotographic processing. Such sensitizers include dyes, for example, those of the sulfophthaline series, the xanthane series and dibasic acids, for example, phthalic acid and its derivatives.

It presently appears that the relatively higher concentration of resinous insulating binder in the first coating tends to act as a better binder to the base, which increases press life of the plate, and also to result in a greater build-up of electrostatic charge in the electrophotographic process, thereby resulting in a denser image. The relatively higher zinc oxide concentration in the second coating, on the other hand, appears to enable easier conversion as well as to provide greater hydrophilicity, which results in clearer backgrounds over longer press runs.

The following examples ae set forth to illustrate the invention more fully and are not intended to limit in any way the scope of the invention as disclosed hereinbefore. The amounts of materials recited in these examples are expressed in parts by weight unless otherwise noted.

EXAMPLE 1 A sheet of smooth-surface paper was coated with a first layer of a dispersion containing 160 parts of zinc oxide (Photox 801 from New Jersey Zinc Co.), 100 parts of an aqueous acrylic copolymer emulsion, Hycar 1800x72, parts of water and about 1 part of a 1% aqueous solution of Bromphenol Blue. A second coating of a dispersion differing from the one described by containing 160, 50, and about 1 part, respectively, of those materials was applied over the first coat. Each coat of dispersion was dried under infrared lamps and the coated sheet was then placed in a dark chamber at 50% relative humidity and room temperature for about 5 hours. The sheet was then exposed electrophotographically to an image using a Model 44 SCM Coronastat co ier which uses a conventional liquid-type developing composition. A conversion etching solution was then used to render the non-image areas hydrophilic by reaction with the zinc oxide at the surface of the coated layer. The solution contained 12 parts each of monoand di-ammonium phosphate, 2.4 parts of glacial acetic acid, 12 parts of sodium ferrocyanide, 12 parts of phosphoric acid, 600 parts of gum arabic and water to make 2400 parts. The printing plate thus prepared was placed on a lithographic press and over 300 high quality copies were obtained. The copies had dense images and clear backgrounds.

A sheet coated only with the first dispersion was exposed, developed and used to print, but the background of the copies tended to darken indicating poor conversion an a relatively oleophilic non-image area. A sheet coated only with the second dispersion tended to produce copies having less dense images.

EXAMPLE 2 A plate was prepared as described in Example 1 using a first dispersion containing 80 parts of an alkyd resin, #6035 made by Cellomer Corporation (about 50% solids), parts of zinc oxide, Photox 801, 200 parts of xylene and a small amount of Bromphenol Blue, and a second dispersion containing 80, 320 and 240 parts, respectively. The plate was then exposed, developed and used on a press as described in Example 1, and produced high quality copies having dense images and clear backgrounds.

EXAMPLE 3 A plate was prepared as described in Example 1 using a first dispersion containing 80 parts of an alkyd resin, #6016 made by Cellomer Corporation, 160 parts of zinc oxide, Photox 801, 200 parts of xylene and a small amount of Bromphenol Blue, and a second dispersion containing 80, 320 and 240 parts, respectively. The plate was then exposed, developed and used on a press as described in Example 1 and produced high quality copies having dense images and clear backgrounds.

EXAMPLE 4 A plate was prepared as described in Example 1 using a first dispersion containing 100 parts of resin No. 13RE57, available from Stein-Hall Company of New York and believed to be predominantly vinyl acetatecontaining terpolymer, 100 parts of zinc oxide, Photox 801, 200 parts of xylene and a small amount of Bromphenol Blue, and a second dispersion containing 100, 250 and 240 parts, respectively. The plate was then exposed, developed and used on a press as described in Example 1, and produced high quality copies having dense images and clear backgrounds. Over 250 copies were made on the press without any peeling of the coated layers in contrast to a sheet coated with only the second dispersion which tended to peel after about 50 copies.

I claim:

1. An article adapted for conversion electrophotographically into a planographic printing plate comprising an electroconductive base, a first photoconductive coating thereon containing powdered zinc oxide in a resinous insulating binder, and a second photoconductive coating on said first coating containing powdered zinc oxide in a resinous insulating binder, the concentration of zinc oxide in said second coating being greater than the concentration of zinc oxide in said first coating.

2. An article as defined in claim 1 wherein the concentration of zinc oxide in said second coating is at least about 50% greater than the concentration of zinc oxide in said first coating.

3. An article as defined in claim 1 wherein the concentration of zinc oxide in said second coating is about 2 to about 4 times greater than the concentration of zinc oxide in said first coating.

4. An article as defined in claim 1 wherein the conl centration of zinc oxide in said second coating is in the range of about 1:10 to 8:1 parts by Weight based on the weight of said binder, and the zinc oxide concentration in said second coating is in the range of about 2:1 to 32:1 parts by weight based on the weight of said binder.

5. An article as defined in claim 1 wherein the concentration of zinc oxide in said first coating is in the range of about 1:1 to 5:1 parts by weight based on the weight of said binder, and the zinc oxide concentration in said second coating is in the range of about 4:1 to 12:1 parts by weight based on the weight of said binder.

6. An article as defined in claim 1 wherein said first coating is up to about 80% and said second coating is up to about 90% of the combined thickness of said coatings.

7. An article as defined in claim 6 wherein said combined thickness of said coatings is less than about 50 microns.

8. An article as defined in claim 6 wherein said combined thickness of said coatings is in the range of about 5 to about 15 microns.

9. An article as defined in claim 1 wherein said base is an electroconductive material.

10. An article as defined in claim 1 wherein said base is treated with an electroconductive material.

11. An article as defined in claim 1 wherein both surfaces of said base are coated with said first and said second coatings.

12. A planographic printing plate comprising a dimensionally stable flexible base, a first layer on said base of powdered zinc oxide in a resinous insulating binder, a second layer on said first layer of powdered zinc oxide in a resinous insulating binder, the concentration of zinc oxide in said second layer being greater than the concentration of zinc oxide in said first layer, an image area on said second layer comprising an oleophilic resinous material, and a non-image area on said second layer comprising a hydrophilic reaction product of zinc oxide.

13. A planographic printing plate as defined in claim 12 wherein said concentration in said second layer is at least about 50% greater than said concentration in said first layer.

14. A planographic printing plate as defined in claim 12 wherein said concentration in said second layer is about 2 to about 4 times greater than said concentration in said first layer.

15. A planographic printing plate as defined in claim 12 wherein said hydrophilic reaction product of zinc oxide is formed by reaction of zinc oxide on the surface of said second layer.

16. A planographic printing plate as defined in claim 12 wherein the combined thickness of said first and said second layers is less than about 50 microns.

17. A planographic printing plate as defined in claim 12 wherein the combined thickness of said first and said second layers is between about 5 and 15 microns.

18. A planographic printing plate as defined in claim 12 wherein said hydrophilic zinc oxide reaction product is a reaction product of zinc oxide and a member selected from the group consisting of phosphates, ferrocyanides, ferricyanides, cobalticyanides, vanadates, molybdates, tungstates, tartarates, oxalates, tannates and citrates.

References Cited UNITED STATES PATENTS 2,987,395 6/1961 Jarvis 961.7 3,165,405 l/ 1965 Hoesterey 961.7 3,309,990 3/1967 Klupfel et al. 101456 XR 3,364,021 l/l968 Hazen 96l.8 2,963,365 12/1960 Greig 961.8 3,001,872 9/ 1961 Kurz 101463 X 3,106,158 10/ 1963 Michalchik 101-462 X ROBERT E. PULFREY, Primary Examiner F. FREI, Assistant Examiner U.S. Cl. X.R.

961.8, 33; 101-457, 462, 465; ll72l8 Tag? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 48] 27] Dated December 2 IQfiQ Inventor(s) Kenichi Shimazu It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- Column 4, line 51, change "an" to and Column 5, line 35, change "second" to first SIGNED AND SEALED MAR 101970 wmnu 1:. W, .m.

Attesting Officer commissioner of Patents 

